How do you decide the diameter of the rotor in a disc brake??

Hello People.. I do the brakes designing for our Mini Baja Vehicle. When I went through the calculations there was nothing like a rotor diameter mentioned anywhere but it looks like when you want to purchase a rotor from a standard company for your vehicle they expect only the "Braking Torqe"... What I would like to know is how is it that they're able to decide the rotor diameter with the help of just braking torque??

Please guys help me in this..

And the blogs' jus awesome and the guy who created this forum.. "YOU ARE AWESOME!!!"

You don't need a reference, draw a free body diagram of the disc and caliper and apply Newtons laws.

You can come at the problem from a different direction.

1. You want to be able to skid the wheels on dry pavement.
that constraint along with knowledge of your cars weight, wheelbase and center of gravity will allow you to calculate brake torque required to skid

2. Generally you want your hydraulic brakes to operate around 1000 psi.
This info and knowledge of your brake caliper piston diameter will allow you to calculate brake clamping force.

As Erich mentions, the relationship I showed was derived from first principles based on a free body diagram of the system. I ignored the diameter of the caliper in the relationship. If you want a reference anyways, I found a paper called "The Disk Brake Design and Performance Analysis." They give the relationship in a more general way:

T = r*u*P*A*n

and

r = D/2 - (d/2+1) //I'm not sure where the "+1" term comes from. Anyone have any idea??